(1) Field of the Invention
The present invention relates to a system and method for controlling ignition timing in an internal combustion engine which are suitable for achieving a smooth increase of engine revolutions during acceleration (deceleration) of a vehicle in which the engine is mounted.
(2) Background of the Art
Various types of engine ignition timing control systems have been proposed in which the ignition timing is controlled so that the engine is driven according to optimum engine operating conditions
In such ignition timing control systems, optimum values of the ignition timing are previously stored so as to correspond to input information representing the engine operating conditions and optimum controlled variables are read in accordance with changing engine operating conditions.
For example, an optimum ignition timing angle is derived through experiment or so on, then, a map table is prepared in which the optimum ignition timing angle is determined using the engine revolution speed and injection pulsewidth. The map table is stored in a ROM (Read Only Memory) installed in a control unit constituted by a microcomputer so that when the engine is driven, the values of the engine revolution speed, detected by means of a sensor, and of the injection pulsewidth are calculated in the control unit and are used to read the ignition timing angle value from the table map to execute the ignition timing control.
However, in cases where a transient state occurs such as engine acceleration/deceleration, a control delay occurs as the result of a required value changing abruptly so that the engine revolution speed is not smoothly increased and fluctuations in the engine revolution speed often occur.
Such revolutional fluctuations are transmitted to a power train as torque variations so that low-frequency vibrations are generated in a vehicle body in forward and reverse directions (referred to as surges). Such surges give the vehicle driver an unpleasant feeling of wobbling or shaking with the vehicle body (referred to as shaking or wobbly vibrations) and reduce driveability of the engine.
To prevent such shaking vibrations, the ignition timing needs to be controlled so as to suppress the engine revolution fluctuations. That is to say, when the engine revolutions drop during the occurrence of shaking vibrations, the ignition timing is advanced to increase the engine torque. When the engine revolutional speed is increased, the ignition timing is retarded to decrease the engine torque. Thus, the ignition timing control system causes the supression of revolutional fluctuations and torque variations.
However, the occurrences of the surges and shaking vibrations are changed according to the engine load state. For example, when an engine throttle valve is largely opened, the ignition timing is corrected so that the engine output torque is largely changed but, in a case of a partial load, the change of the engine torque hardly occurs even if the ignition timing is corrected.
With the above-described problem in mind, a Japanese Pat. Application sho No. 62-280922 (which is not yet published) filed on Nov. 9, 1987 discloses an ignition timing control system in which the load state on the engine is taken into account. The ignition timing control system is corrected so as to further suppress the occurrence of shaking vibrations. In detail, when the engine enters a transient state, the ignition timing control system sets a first correction coefficient to correct the ignition timing according to the engine revolution speed and sets a second correction coefficient to correct the ignition timing based on the engine load condition so that the revolutional fluctuations during acceleration are suppressed according to the engine load conditions to prevent occurrence of the shaking vibrations.
However, since the ignition timing control system disclosed in the above-identified Japanese Patent Application corrects the ignition timing on the basis of the engine load and engine revolution speed; when the engine falls in the transient state, the quantity of ignition timing advance angle (or retardation angle quantity) at a time an accelerator pedal is depressed with a transmission gear position placed in a first-speed gear ratio becomes insufficient. When the vehicle driver depresses the accelerator pedal in an engine idling state (i.e., the gear position is in a neutral gear range or the clutch is disengaged), the ignition timing control is continued. Hence, the advance angle quantity becomes excessively increased so that a smooth increase of the engine revolutional speed cannot be felt by the vehicle driver when the vehicle transfers from a stopped state to a running state.
That is to say, since the engine torque at a time when the first-speed gear range is placed is transmitted to the vehicular power train, multiplied by a gear ratio and the vibrations in the engine torque are transmitted as vibration applying forces, an attenuation quantity of the engine torque is reduced due to a normal ignition timing correction so that the shaking vibrations cannot sufficiently be suppressed. In addition, since the engine torque is not transmitted to the vehicular power train when the gear position is placed in the neutral position or when the clutch is disengaged, the corrective control of the ignition timing is not necessary at the time of the vehicle stop and, thereafter, the feeling of smoothly increasing the engine revolution speed cannot be felt by the vehicle driver due to a response delay of the ignition timing control when the engine is transferred into a running state from a vehicle stop in the idling state.